The present invention relates to a biasing technique for a laser transmitter and, more particularly, to using the envelope of an applied RF signal to dynamically alter the bias current applied to the laser device.
Cable TV systems, also known as Community Access Television (CATV) Systems, have evolved from simple broadcast systems for television services to bi-directional broadband networks that can carry voice, video and data traffic. This evolution has been accomplished in part by upgrading traditional cable networks to Hybrid Fiber Cable (HFC) networks which utilize fiber optic systems in conjunction with active electronics and coaxial cables to deliver broadband signals to the home. These networks also support the reception of return path signals, defined as signals generated by units in or near the subscriber and which send data or voice signals from the subscriber or business location to the network through the cable system.
In the common HFC architecture, a fiber optic transmission system is disposed at the head-end and is used to transmit xe2x80x9cdownstreamxe2x80x9d (i.e., broadcast) signals toward the various subscribers/business. At predetermined locations, the optical fibers are coupled to sets of coaxial cables, using a xe2x80x9cfiber nodexe2x80x9d, where the coax transmission paths then carry the signals along the remainder of the path to the individual locations. On the return path, therefore, the upstream signals first travel along the coax cables and are then combined at the fiber nodes, converted to optical signals, and transmitted up through the remainder of the fiber network to the headend. A laser transmitter in each fiber node is used to convert the received upstream electrical signal(s) into an optical signal.
In conventional fiber nodes, the bias current applied to an upstream laser is either held constant or automatically adjusted so as to keep the laser output power constant. As mentioned above, upstream rf signals from cable modems, set-top-boxes, and other customer premise equipment all converge on the fiber node as they exit the coax network, where these combined electrical signals are then used to modulate the laser driver in the fiber node. Since the modulating signal level can be quite variable (depending upon the number and strength of the individual return signals), the bias level applied to the laser must be sufficiently high so as to ensure that the modulating signal will not excessively clip the laser. As is well-known in the art, clipping-induced distortion of a laser driver will result in bit errors in the optical output. In the particular broadband communication system environment, the magnitude of the signal modulating the laser will vary rapidly as various modems in the network burst xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d, and will also vary gradually as new channels are allocated or as the physical characteristics of the network change. The variability in the RF signal level can therefore be quite large. Moreover, the RF level of each channel may not be equal and, within a given channel, excessive loss between a particular cable modem and the fiber node may result in bursts from that modem being weaker than those from other modems. Ingress noise can also contribute to the loading of the laser driver in an unpredictable way.
When designing and installing a HFC system, there must be enough xe2x80x9cmarginxe2x80x9d in the laser bias at each fiber node to account for all of these sources of variability in the upstream transmission paths. Accurately characterizing or predicting this variability on a node-by-node basis is not trivial or, in fact, even reliable if the network is subject to constant change by the addition or deletion of customer premise equipment. Thus, even more margin may be built into a chosen bias level to simplify and standardize the system design process, since insufficient margin in the presence of new services may result in unforeseen link degradation (as a result of laser clipping).
The simplistic solution of providing additional margin, however, comes at a cost. The optical modulation depth (OMD) per channel is lowered, and the extra optical power leads to extra relative intensity noise (RIN), as well as shot noise at the receiver. In systems with either optical or RF combining in the upstream, the excess RIN and shot noise from the set of lasers will accumulate. Moreover, in a passive optical network (PON), optical beat interference (OBI) can also be a source of noise if the laser wavelengths overlap, where the OBI noise-power spectral density is proportional to the convolution of the optical spectra. When the OMD is low (as is the case for high margin), the impairment from OBI will be worse. In particular, since the OBI depends on the average optical power in each interfering laser, the carrier-to-noise ratio (CNR) is maximized when the OMD is maximized. Secondly, modulating the laser broadens its linewidth, spreading the OBI spectrum and reducing the noise that falls in any given channel.
Thus, a need remains in the art for an arrangement capable of biasing a laser transmitter (particularly in an HFC broadband network environment) that prevents clipping of the laser, while maintaining the margin at a reasonable level.
The need remaining in the prior art is addressed by the present invention, which relates to a biasing technique for a laser transmitter and, more particularly, to using the envelope of an applied RF signal to dynamically alter the bias current applied to the laser device.
In accordance with the present invention, an envelope detector is included in a laser transmitter and is responsive to the incoming RF signal. Envelope biasing in accordance with the present invention requires the passband signal modulating the laser to be sub-octave. An up-converter is included in the detector arrangement to shift in the incoming RF signal (in the 5-42 MHz band, for example) by a predetermined carrier frequency fc into a sub-octave band using single-sideband (SSB) modulation. The laser driver is then biased by a minimal dc amount plus the detected envelope signal, thus providing a dynamic bias that is just sufficient to prevent laser clipping. Alternatively, double-sideband (DSB) modulation may be used to up-convert the received RF signal.